Quicktime File Format (2012-08-14).pdf

Profile Atom GuidelinesProfile Atom SpecificationFeature value ranges should in general never include an unknown point; if the value of a feature is unknown,the feature should be absent from the profile atom.Feature values should be deducible by fairly simple inspection of the rest of the movie: for example, extractingthe profile-level ID from a video header, or calculations using information from the sample table (for example,overall average bit rate by summing the sample sizes and the sample durations). It is not appropriate to havefeatures which cannot be computed, or only computed with difficulty (e.g. a buffer model estimation whichrequires emulating a video decoder on the entire bit stream). The algorithm to extract or deduce the featurevalue from the rest of the file must be defined.Empty slots in the profile atom structure must be filled with zeroes.If there are multiple parts of the file to which the same feature apply, yet they have different feature values,then either there must be entries for each occurrence or none at all. For example, if there are two MPEG-4visual sequences, using different visual profiles, there are either two profile entries in the profile table (one foreach sequence) or none at all. Features must not be partially documented.Profile atoms may also occur at the track level. A track-level profile atom must occur within the track atombefore the track header atom ('tkhd'). A reader should stop searching for a track’s profile atom if either theprofile or the track header atom is found, ignoring any other atoms present.A track profile atom should only summarize features within that track. If track profile atoms exist, a movieprofile atom can be built largely by copying feature entries from the profile atom of the movie’s tracks to theprofile atom at the movie level. It is possible to have multiple track profiles with different values which mustbe resolved to a single value for the movie as whole, however—such as multiple video tracks with differentmaximum bit rates—so not all features can be copied directly from the track to the movie profile. Additionally,the movie profile may summarize features that cannot occur at the track level, such as total movie bit rate.When building a movie profile, you must include either all instances of a track-level feature or no instances ofthat feature. For example, if you have multiple video tracks that use different codecs, you must either includean entry at the movie level for each codec, or put no codec feature entries at the movie level at all.2012-08-14 | © 2004, 2012 Apple Inc. All Rights Reserved.395